A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Fuel cells are a clean and efficient power source, and may replace traditional power sources such as the internal combustion engine in both stationary and automotive power applications.
In a polymer electrolyte membrane (PEM) fuel cell, the electrolyte is a solid polymer membrane which is electronically insulating but ionically-conducting. Proton-conducting membranes based on perfluorosulphonic acid materials are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a polymer electrolyte fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymer membrane. On either side of the membrane there is an electrocatalyst layer, containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode. Finally, adjacent to each electrocatalyst layer there is a gas diffusion substrate. The gas diffusion substrate must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the substrate must be porous and electrically conducting.
The MEA can be constructed by several methods. The electrocatalyst layer may be applied to the gas diffusion substrate to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of a membrane and laminated together to form the five-layer MEA. Alternatively, the electrocatalyst layer may be applied to both faces of the membrane to form a catalyst coated membrane. Subsequently, gas diffusion substrates are applied to both faces of the catalyst coated membrane. Finally, an MEA can be formed from a membrane coated on one side with an electrocatalyst layer, a gas diffusion substrate adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Field flow plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs, removing products, providing electrical connections and providing physical support. The field flow plates and MEAs in the stack are compressed together at pressures typically from 50 to 200 psi absolute, using for example a bladder or piston system or a series of bolts located in stack end plates. Typically, one of the stack end plates also contains the necessary ports to provide access and removal from the stack of the reactants, products and any associated humidification water. Ports are also required to provide access to and removal of the stack coolant from the stack cooling plates which are needed to remove the excess heat generated within the MEAs. From the ports in the stack end plate the gases and fluids are transported through the stack to each field flow plate. The porting design may require that internal sections of each MEA are removed or all the porting may be external of the MEAs.
In the fuel cell stack it is essential that any loss to the atmosphere or any potential mixing of the reactants is prevented. This would lead to a decrease in the overall system efficiency and is a potentially hazardous situation due to the risk of combustion from the mixing of the fuel and oxidant. Sealing and gasketing of the components in the stack are used for the purpose of preventing such occurrences. For the purposes of this description, the term “sealing” is used to denote a method of preventing fluid diffusion out of or through a single component. For example, the perimeter of a gas diffusion substrate can be sealed by impregnating the perimeter with a sealant material. The term “gasketing” is used to denote a method of preventing fluid diffusion between components by placing a resilient material between the two components.
In a well-known method of sealing and gasketing the components in a fuel cell stack the membrane protrudes beyond the gas diffusion substrates by a considerable margin, e.g. by as much as 25 mm, so that gaskets can be positioned between the protruding membrane and the field flow plates. The gaskets are held in place by compression. This method can be problematic, particularly with the very thin membranes (approximately 30 μm) that are increasingly being used, because the membrane is weak and may be damaged by the compressive forces. Additionally it is wasteful to use large amounts of expensive membrane material in regions outside the active area of the membrane electrode assembly.
Membrane electrode assemblies are generally not very strong, and an assembly with a protruding membrane will have a particularly weak edge region. This can cause difficulties when handling membrane electrode assemblies, and when constructing the fuel cell stack.
U.S. Pat. No. 5,187,025 describes a membrane electrode assembly wherein the problem of gasketing directly onto the membrane is avoided and wherein the strength of the edge region of the membrane electrode assembly is improved. A plastic spacer surrounds the membrane and plastic films with an adhesive layer are bonded to both sides of the spacer and the membrane. This provides a rigid frame around the membrane and effects a gas tight seal around the membrane edge. WO 00/74160 discloses another membrane electrode assembly wherein a reinforcing frame is provided by plastic films with adhesive layers that are bonded to the membrane. The adhesive layers extend beyond the membrane and a strong bond is formed between the two adhesive layers.
In these previous examples the plastic films and adhesive layers are bonded to the membranes. Membrane materials undergo changes in size depending on their level of hydration. If the membrane is constrained by a rigid frame formed by plastic films and adhesive layers, changes in hydration may lead to stress on the membrane and possible damage.
WO 00/74160 discloses that the plastic films can be embedded in the gas diffusion substrates, but there is no disclosure of sealing or reinforcing the edge of a membrane electrode assembly by applying the plastic film and adhesive layer to the substrate.